The invention relates to a directional coupling device for multi-mode optical fibers, which comprises a coupler body made of a material which is transparent in the wavelength range of optical radiation and which is used as a carrier for the transmitted information, and which body is destined to co-operate with the ends of two optical fibers, at least one of these fibers transferring the information from one or more information sources which are disposed along the information routing circuit to which said optical fibers belong, and at least one of said fibers transferring the information conveyed by the other fiber, to which the latter information specific information is added which is introduced in the directional coupling device.
Coupling devices of the type are known, for example from an article published in the American magazine "Electronics" of Dec. 20, 1973, (page 30), entitled: "Simple coupler taps fiber optic cables".
The optical coupler described in this article is made by diagonally cutting a small parallelepiped-shaped block of glass, polishing the two surfaces which correspond to the plane of cutting,. silvering one of the surfaces thus prepared while leaving a transparent pupil in the center of said surface, and subsequently reassembling the block.
If the light coming from a bundle of optical fibers is concentrated at one of the ends of the coupler, a small fraction of the light beam is reflected by the silvered surface and propagates perpendicularly to the axis of the coupler towards one of the sides of the block, where it can be detected by a photodiode. Most of the light, however, continues through the coupler towards the other end, where a fraction of the light re-enters the bundle of fibers which terminates at said other end. For introducing information, a light-emitting diode emits radiation perpendicularly to that side of the block, which is opposite to said side, in the direction of the other face of the silvered area.
The coupling device whose structure and principle of operation are described in the cited article, is used in co-operation with cables consisting of a bundle of multi-mode fibers and is consequently not suitable for use with single multi-mode fibers at the input and at the output of the coupling device.
The manufacture of such a coupler is difficult and complicated. If it would be necessary to reduce the dimensions of the coupler in order to adapt it for operation with a multi-mode fiber at the input of the coupler and a multi-mode fiber at the output of the coupler, the manufacturing problems would become substantially insurmountable. The necessity of extremely accurate arrangement and fixation of the optical fibers relative to the pupil of the mirror, which is integrated in the tiny parallelepiped block of the coupling body, would present difficult problems to the user.
Furthermore, the insertion losses introduced by the coupler described are substantial (4.5 dB at the moment that the article was written, with the expectation that this could be reduced to 3 dB), while both the degree of coupling between the bundle of optical fibers and the percentage of luminous flux needed to enable information to be extracted are not adjustable.